A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, use is made of stages, such as a substrate stage or a patterning device stage that is movable so as to position the substrate respectively the patterning device, which stage is provided with a cable connection, e.g. to provide electric power to actuators (such as a short stroke motor), sensors (such as encoder measurement heads mounted on the stage), etc. The cable, due to its inherent properties such as its stiffness, may contribute to sources of mechanical disturbance when positioning/moving the stage with high accuracy and/or high speed. Therefore, it may be desirable to be able to provide a stage in which wired connections are omitted as much as possible.
Clock data may be transmitted to the stage. The clock data may for example be used to clock (i.e. to time) position measurements of the stage, e.g. during movement of the stage. In case the stage is equipped with encoder measurement heads, the measurements by the encoder measurement heads may for example be clocked. Given a range of movement of the stage, a wireless transmission of clock data to the stage would result in varying delay of the wireless transmission of the clock data: as a propagation path of the transmission would vary depending on a length of the transmission path. As a result, the clock data as received at the stage would exhibit a delay in dependency of the position of the stage. Given high speeds of movement of the stage and high positioning accuracy requirements, it may be desirable to determine a position of the stage—e.g. during a movement—with high accuracy in terms of the position itself, as well as in terms of the time at which that position is measured. Thereto, an accurate clock may be desirable.